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The Forensic Examination Of Video Tape--Technical, Integrity And Legal Issues

A recent publication of VideoMaker Magazine cites that approximately 30 million
people have purchased consumer-type camcorders and that this number is increasing
exponentially and greatly exceeds the number of professional cameras involved in the
broadcast industries.(1)

As this ready availability of consumer camcorder equipment continues to increase, the
likelihood exists that even larger numbers of VCRs will be purchased not only for playback
of the video image but more recently to provide off-line (non-computer based) editing of
original videotapes. The purpose of this article is to review some of the basics of
videotape recording, the editing of tape and its detection by the forensic expert,
and the continuing and increasing threat posed by both linear and non-linear (on-line or
computer based) editing to both the professional and consumer videographer.

Technical Issues

Analog Recording vs. Digital Recording

Storage of information for both audio and video tape recording is accomplished using
either analog magnetic tape or in the case of the computer, high density digital computer
disks (or diskettes).(2) During playback, the recorder senses the different voltage
changes, converts the electrical signals, and selectively filters the resulting electrical
signals which actuate the electron guns or amplifiers of televisions or video monitors.
The actual conversion of the electrical output signals occurs by passing the videotape in
front of a rapidly rotating helically scanning magnetic head. An analog video signal
transmits light and color information by means of both measuring and displaying voltage
amplitude and frequency differences of the signal presented.(3) In digital recording, the
combinations of differing high and low voltages are stored as a series of zeros and ones
of binary code understandable to computers. A digital recorder reproduces the audio and
video signals through their binary code conversions. Due to its excellent reproducibility
and the tapeless medium aspect of digital audio and video recordings, this technology will
soon become the predominant form of recording.(4)

The most popular videotapes presently in use include VHS, VHS-C, and 8 mm although
greater resolution and quality of film can be obtained in the Hi-8 and S-VHS varieties of
film. The consumer market lately has been adopting "multi-media" or
"desk-top" video work stations into the recording and playback of video
information. The more popular work stations provide for a combination of both analog
recording and digital editing at a fraction of the cost that was possible as little as
five years ago. The computer can also be used as a "edit controller" which
instructs which analog tape decks to play and record thus allowing a composite videotape
to be produced which incorporates portions of different source recordings.

Time Code and Timing Errors

Numerous VCRs present instabilities during both the recording and playback of the
videotape which often causes timing differences or time-based errors in the video signal.
Time code is an electrical signal (usually eight digits in length) which tracks individual
frames for the purpose of editing. A troublesome feature of time code is that its
appearance on the tape does not mean that it was originally generated by the original
equipment. Time code can also be employed to "stripe" the tape which means
basically over-recording or erasing the original time code and then add additional time
codes through the use of an external time code generator. As digital editing becomes more
widespread, time codes of all types will become more vulnerable to tampering since like
all information on the videotape they can be reduced to easily manipulate binary code.

Essentially it is the mechanical instabilities of the recording and playback camcorders
and VCRs that cause timing differences which produce anomalies in the horizontal, vertical
and color synchronization pulses. Slight variations in the synchronization pulses tend to
cause vertical jumping, horizontal wavering, and color and hue saturation differences in
the video signal. Added to these difficulties are tape stretching and tension difficulties
as the tape wraps around the respective video heads.

Often these "time-based" errors in synchronization and other signal anomalies
permit the forensic expert to help individualize the camcorder and/or VCR recorders. (5)

Integrity Issues

Analog (Linear) vs. Computer Based (Non-Linear) Editing

Editing normally involves: 1) real-time editing which is done simultaneously with the
making of the video footage, and also commonly known as "in-camera" editing; 2)
editing done subsequently to the production of the video tape which does not affect the
source original tape, and 3) post-production editing which is done subsequent to the
original shooting but which does affect the original source recording. Contemporaneous or
real-time editing normally involves the videographers subjective decisions as to
who, what, when, and where to record. This is achieved by interrupting the record events
through stopping, pausing, restarting and rewinding or over-recording a portion of the
original tape. Depending upon the type of camcorder used, this form of editing can be seen
as a natural part of the recording process and not necessarily used to distort or falsify
evidence recordings. Post-production editing can also be legitimately accomplished through
a variety of analog and digital recorders, especially if this type of editing has been
authorized by appropriate judicial authorities. It is the context in which the editing
occurs which determines whether intentional falsification occurs. Falsification normally
has two distinct meanings: 1) intentional alteration wherein the videographer intends to
deceive the viewer is known as "falsification" or 2) "alteration"
which is any subsequent change made to the original recording. The latter category may not
be suspicious, however, if it involves real-time editing or the re-recording of unaltered
copies, especially at the request of the attorneys.

The two major categories of alteration include: 1) that associated with splicing,
erasure and re-recording where changes do occur to the original source recording and; 2)
when an altered copy or an enhanced copy (either audio or video) is re-recorded with no
physical change done to the source recording. In other words, the actual alteration that
is observed occurs always in the re-recorded tape or "edited master", and not on
the original source recording. Examples of non-source affected changes include deletion of
prejudicial or objectionable material; rearrangement of the video content for
demonstrative purposes, or edition of video material not present in the source
recording.(6)

In the simplest type of tape editing, one need only use a camcorder, a VCR, and the
appropriate connecting cable to affect the edit. Most inexpensive editing is accomplished
through connecting the VCR to the camera which has most of the editing functions , ie.
Flying erase head, special effects, time-code and other analog and digital camera options.

"Assemble Editing" basically involves laying down one scene after another. It
requires that you locate the last frame if that event you wish to keep and hitting the
pause button on the VCR at the edit point. With the camcorder in pause mode, you would
then press the record button to toggle the record/pause mode thus allowing the camcorder
to begin assembling respective edits.

A different type of editing is known as "insert" editing and unlike the
aforementioned assemble editing does affect the audio portion of the program. In this form
of editing, part of previously shot footage is replaced with new video, but basically
leaves the audio portion intact. Both of these types of simple editing can result in
errors including timing, phase and synchronization problems which would produce glitches,
smears, rolls or other artifacts consistent with consumer-type editing.

Audio-dubbing is the opposite of video-dubbing in that it replaces the audio without
touching the video. If your camcorder does not contain an "audio dub" feature
for insert dubbing, you can attach an audio mixer between the audio connection of your
source VCR and the recording camcorder. Lately many VCRs also include both audio and video
dubbing features together with other editing options. If glitch-free edits are desired, it
is necessary for the videographer to use flying erase heads, audio dubbing keys, video
insert features and a jog/shuttle control knob to provide accurate methods of locating
precise frame locations.(7)

Many videotape formats have at least two separate audio tracks which lend themselves to
fairly easy manipulation during any tampering effort. Telltale signs of the editing
process may include poor synchronization with a new narration seen to exist with the video
component and ambient noise levels. The audio signal in videotape is generally much less
complex and contains less information than the video signal and is much easier to
manipulate or fabricate. Using audio dubbing technology, it is possible to completely
rearrange words, sounds and sentences or to produce audio segments with unintended,
opposite and legally detrimental meanings. Audible signs that may reflect editing would
include significant changes in volume, content, or continuity with either the main
speakers words or background sounds, sudden or strange sounds, and the audio
component not fully synchronized with the relevant video picture.

Computer-Based (Non-Linear) Editing Systems

For approximately $400 one can now purchase a video capture card while the more
expensive varieties provide for the reading of a variety of time codes to include
additional effects as a titler, audio mixer, and special effects generator. Currently a
complete windows-based non-linear editing system costs approximately $2500 to $3000 which
includes a 133 mhz CPU, 16 megabyte of RAM, and a 1.6 gb hard drive together with capture
and display of video images.(8)

Using digital technology, the consumer videographer is now more capable of
falsification and fabrication of both still and video motion images. The telltale glitches
or other edit artifacts often associated with cut and paste editing or assemble editing
are rapidly being overcome as the digital video work station becomes more powerful and
accessible to the community. Obviously this type of seamless editing poses an increasing
threat to tape admissibility, especially if authenticity questions arise out of its
manufacture.

Detection of Editing

With the advent of more sophisticated analog and digital editing devices, it is
possible that detection of intentional edits will become more difficult if not impossible
to identify. To aide the forensic expert in the detection process a variety of
instrumental tools are available. These include a cross-pulse monitor, frequency
generators, video signal generators, oscilloscopes, waveform monitors/vectorscopes,
alignment tapes, and magnetic developing solutions.

The waveform analyzer/vectorscope can assist in troubleshooting problems involving
luminance, chrominance and audio stages of the VCR which allegedly was used to record the
original tape. For example, certain waveform monitors can measure anomalies in the servo
motors; detect capstan speed errors; measure capstan jitter; drum speed errors; and
calculate inconsistencies in the drum speed movements (ie. drum jitter). If the camcorder
is available for inspection, additional video analyzing devices allow for measurement of
deficiencies in the cameras video signals, (luminance and chrominance) and also
color aberrations. Additional tests include the measurement of power adapter/power supply
problems, video and chroma (noise) artifacts, "burst frequency", and
"frequency error problems." Lastly, magnetic development of the original videotape
can often visualize important audio and control track timing and misalignment information.
For example if an 8mm original tape was subsequently subjected to the full-width erase
head action of a VCR, the resultant erase head signature could be visualized on the video
microscope and documented if authenticity questions subsequently arose.

Legal Issues

Material alterations to video tape can either be done intentionally or accidentally. If
accidental, the videographer should provide a detailed description of the circumstances
surrounding his unintentional editing of the original tape. If his actions to not affect
the reliability or trustworthiness of the evidence, the recording can still be admitted.
Authorized judicial "editing" of the tape often involves the removal of
irrelevant or prejudicial content materials; privileged and confidential conversations;
enhancing the image for jury review. If the editing is accomplished intentionally or
surreptitiously, then it is within the trial judges discretion to allow the
evidence, although many courts are looking more and more at this type of evidence
unfavorably.(9)

If questions still arise concerning the recordings authenticity, then discovery
actions and preliminary interrogations would normally ensue by the attorneys and would
include anything that bears on the relevance, authenticity, and possible incompetency of
the video evidence including:

When and how the videotape was produced

Was the opposing party in control of its production

How and when did the tape come into their possession

The identity of the videographer and his credentials

Identity of authentication witnesses

Whether in-camera or post-production editing was accomplished and how

Does opposing party plan to use it for substantive or illustrative purposes

Attempt to obtain all relevant footage including "out takes".

Request all recording equipment including VCRs, cameras, cabling materials, microphones,
and any other materials used in the taping process.

NOTE: IF THE ORIGINAL RECORDING EQUIPMENT AND TAPE IS AVAILABLE FOR EXAMINATION, THE
EXPERT SHOULD BE ABLE TO MAKE THE FOLLOWING DETERMINATIONS:

If the recording is an original or copy.

Which record buttons were depressed in the process of making the

recording and where they are relative to the video images being presented.

If part of the original recording was masked by over-recording.

If the recorder presented is actually the one used to make the tape evaluated.

Which of a limited number of recorders was used to make a video recording.

Other case-specific questions that may be appropriate.

As the ability to manipulate video/audio evidence becomes more widespread and
effective, the historical admissibility standards will be rendered inadequate. Likewise as
"undetectable" video editing becomes more prevalent, it is likely that future
video evidence may be excluded if the slightest hint of falsification exists.(10)

--Under "pictoral communication" theory, testimony
by a percipient witness, not necessarily the videographer, that the video recording is a
"fair and accurate portrayal" of what it is purported to represent.

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